"Legal" is describing the Schengen Visa Code, which dictates that visa applications are ordinarily to be processed within 15 days, unless more time is needed:
Decision on the application
Applications shall be decided on within 15 calendar days of the date of the lodging of an application which is admissible in
accordance with Article 19.
That period may be extended up to a maximum of 30 calendar days in individual cases, notably when further scrutiny of the application
is needed or in cases of representation where the authorities of the
represented Member State are consulted.
Exceptionally, when additional documentation is needed in specific cases, the period may be extended up to a maximum of 60
Simply, 15 days is the standard processing time defined by European law.